1. Field of the Invention
The present invention relates to a battery charger for charging batteries, such as nickel-cadmium batteries and nickel-hydrogen batteries, which are used as power sources for hand-held electric devices, such as cordless power tools.
2. Description of the Related Art
It is essential for battery chargers to determine, during a charging process, that a battery has reached a fully charged condition and to stop charging the battery at the time thereof. One method of determining the fully charged condition is a so-called xe2x80x9cxe2x88x92xcex94V detection methodxe2x80x9d, in which the battery is determined to be fully charged when the battery voltage drops a predetermined voltage (xcex94V) from the peak level. To this end, the voltage Vin across the battery is sampled at every predetermined period, and whenever the detected battery voltage Vin exceeds the ever occurring maximum voltage, the data recorded as the maximum voltage is updated. When the battery voltage Vin drops the predetermined voltage xcex94V from the maximum voltage, the battery is determined to be fully charged.
Japanese Patent Application Publication (Kokai) No. HEI-7-184329 describes a xe2x80x9ctwo-step detection methodxe2x80x9d, in which the battery is determined to be fully charged when the battery voltage change exceeds a first predetermined critical value K, and thereafter falls below a second predetermined critical value S. To this end, the voltage across the battery is sampled at every predetermined period, and a previously detected voltage is subtracted from the newly detected voltage to obtain a gradient of the battery voltage, i.e., a battery voltage change. Generally, as the charging progresses, the battery voltage change increases, reaches the peak, and then goes down. The peak of the battery voltage appears at some point while the battery voltage change gradually goes down after having reached the peak of the voltage change. Therefore, by identifying the point with a particular value of the battery voltage change (i.e., the second predetermined critical value) corresponding to the peak of the battery voltage, the fully charged condition of the battery can be known.
Generally, there is less danger that the batteries will be overcharged with the two-step detection method than with the xe2x88x92xcex94V detection method. However, the accuracy in detection of the fully charged condition with the two-step detection method is not assured if there is a change in the charge characteristics of the batteries.
Batteries subject to charging with the charger include active batteries and inactive batteries, which are classified in terms of the use circumstance. Batteries that are normally discharged through the use with electrically powered products are herein referred to as active batteries. Inactive batteries are such batteries that have been left unused for a long period of time, causing the battery to discharge. Active and inactive batteries exhibit different charge characteristics. Further, nickel-hydrogen batteries and nickel-cadmium batteries also exhibit different charge two characteristics.
When the battery charger with the first and second critical values K and S set for the inactive battery is used for charging the active battery, the fully charged condition of the active battery will be detected after the battery voltage peak level has passed, resulting in overcharging the active batteries. On the other hand, when the inactive battery is charged with the first and second critical values K, S set for the active batteries, there is a possibility that the voltage change never reaches the first critical value K, resulting in overcharge of the inactive batteries. Even if the first critical value K is reached, the second critical value S may be reached before the battery voltage reaches the peak level, resulting in undercharging the inactive batteries.
It is an object of the present invention to provide a battery charger capable of reliably and accurately charging batteries to a fully charged condition regardless of the type and kind of the battery.
In order to achieve the above and other objects, there is provided a battery charger that includes supply means, battery voltage detection means, memory, updating means, calculating means, first detection means, second detection means, and determining means. The supplying means is provided for supplying a charge current to a battery. Generally, the battery voltage increases as the charging progresses. The battery voltage detection means is provided for periodically detecting the battery voltage. At least one battery voltage that has been previously detected by the battery voltage detection means is stored in the memory. The data stored in the memory is updated by the updating means whenever the battery voltage is newly detected by the battery voltage detection means. The difference between the newly detected battery voltage and the battery voltage stored in the memory is calculated by the calculating means whenever the battery voltage is newly detected by the battery voltage detection means. The difference is calculated by subtracting the previously detected battery voltage from the newly detected battery voltage.
The first detection means is provided for detecting that the value of the difference calculated by the calculating means has become equal to or exceeded a first critical value and a second critical value greater than the first critical value. The second detection means is provided for detecting that the value of the difference has become equal to or less than a third critical value after the difference had become equal to or exceeded the first critical value. The second detection means further detects that the value of the difference has become equal to or less than a fourth critical value greater than the third critical value after the difference had become equal to or exceeded the second critical value. The determining means determines that the battery has reached a fully charged condition (1) when the second detection means detects that the value of the difference has become equal to or less than the third critical value after the first detection means detects that the value of the difference has become equal to or exceeded the first critical value, or (2) when the second detection means detects that the value of the difference has become equal to or less than the fourth critical value after the first detection means detects that the value of the difference has become equal to or exceeded the second critical value.
According to another aspect of the invention, there is provided a battery charger that includes supplying means, battery voltage detection means, memory, and updating means as described above. The charger further includes first calculating means, second calculating means, first detection means, second detection means, and determining means.
The first calculating means calculates a first difference between the newly detected battery voltage and the battery voltage stored in the memory, and calculates the first difference whenever the battery voltage is newly detected by the battery voltage detection means. The second calculating means calculates a second difference between the first difference and an ever occurring minimum value of the voltage change, and calculates the second difference whenever the battery voltage is newly detected by the battery voltage detection means.
The first detection means is provided for detecting that a value of the second difference calculated by the second calculating means has become equal to or exceeded a first critical value and a second critical value. The second critical value is greater than the first critical value. The second detection means is provided for detecting that a value of the first difference has become equal to or less than a third critical value after the second difference had become equal to or exceeded the first critical value. Further, the second detection means detects that the value of the first difference has become equal to or less than a fourth critical value greater than the third critical value after the second difference had become equal to or exceeded the second critical value. The determining means determines that the battery has reached a fully charged condition (1) when the second detection means detects that the value of the first difference has become equal to or less than the third critical value after the first detection means detects that the value of the second difference has become equal to or exceeded the first critical value, or (2) when the second detection means detects that the value of the first difference has become equal to or less than the fourth critical value after the first detection means detects that the value of the second difference has become equal to or exceeded the second critical value.
According to still another aspect of the invention, there is provided a battery charger that includes supplying means, battery voltage detection means, memory, and updating means as described above. The charger further includes first calculating means, second calculating means, first detection means, second detection means, and determining means.
The first calculating means calculates a first difference between the newly detected battery voltage and the battery voltage stored in the memory. The first calculating means calculates the first difference whenever the battery voltage is newly detected by the battery voltage detection means. The second calculating means calculates a second difference between the first difference and an ever occurring minimum value of the voltage change. The second calculating means calculates the second difference whenever the battery voltage is newly detected by the battery voltage detection means.
The first detection means detects that a value of the second difference calculated by the second calculating means has become equal to or exceeded a first critical value and a second critical value greater than the first critical value. The second detection means detects that a value of the second difference has become equal to or less than a third critical value after the second difference had become equal to or exceeded the first critical value and that the value of the second difference has become equal to or less than a fourth critical value greater than the third critical value after the second difference had become equal to or exceeded the second critical value. The determining means determines that the battery has reached a fully charged condition (1) when the second detection means detects that the value of the second difference has become equal to or less than the third critical value after the first detection means detects that the value of the second difference has become equal to or exceeded the first critical value, or (2) when the second detection means detects that the value of the second difference has become equal to or less than the fourth critical value after the first detection means detects that the value of the second difference has become equal to or exceeded the second critical value.
According to further aspect of the invention, there is provided a battery charger that includes supplying means, battery voltage detection means, memory, and updating means as described above. The charger further includes calculating means, first detection means, second detection means, and determining means.
The calculating means calculates a difference between the newly detected battery voltage and the battery voltage stored in the memory. The calculating means calculates the difference whenever the battery voltage is newly detected by the battery voltage detection means. The memory further stores an ever occurring maximum value of the difference.
The first detection means detects that the maximum value has become equal to or exceeded a first critical value and a second critical value greater than the first critical value. The second detection means detects that the difference has fallen a third critical value or a fourth critical value greater than the third critical value from the maximum value. The determining means determines that the battery has reached a fully charged condition (1) when the second detection means detects that the difference has fallen the third critical value from the maximum value after the first detection means detects that the maximum value had become equal to or exceeded the first critical value but not exceeded the second critical value, and (2) when the second detection means detects that the difference has fallen the fourth critical value from the maximum value after the first detection means detects that the maximum value had become equal to or exceeded the second critical value.